Among metal oxide powders, those having a large specific surface area and a large pore volume show excellent thermal insulation performance. In particular, those produced by drying a liquid in a gel product while inhibiting shrinkage thereof which is caused by the drying (i.e. drying shrinkage) are called aerogels, and are favorably employed as materials for various thermal insulation purposes.
The aerogels are materials with high porosity and excellent thermal insulation properties. Herein, the aerogels refer to solid materials with a porous structure having air as a dispersion medium, and specifically refer to solid materials having a porosity of 60% or more. The porosity represents the amount of air in an apparent volume of a material by a volume percentage. Heat transfer in a material involves solid conduction (transmission of thermal vibrations), convection, and radiation, and in general, contribution of convection to heat transfer is largest in a material having a large porosity. In contrast to this, since the aerogels have a very small pore size of about 10 to 100 nm, transfer of the air through voids is greatly obstructed and heat transfer by convection is significantly inhibited. As such, the aerogels have excellent thermal insulation properties.
For example, a method for producing a silica aerogel is known in which a hydrolysis product of alkoxysilane employed as a starting material is subjected to polycondensation to afford a gelatinous compound, and the gelatinous compound is dried under a supercritical condition of a dispersion medium (Patent Document 1). Another method for producing a silica aerogel is also known in which an alkali metal silicate as a raw material is contacted with a cation exchange resin or a mineral acid is added to the alkali metal silicate as a raw material, to prepare a sol; and after gelation of the sol, a gel afforded thereby is dried under a supercritical condition of a dispersion medium (Patent Documents 2 and 3).
The known methods described above allow manufacture of aerogels with high porosity, by drying and removing the dispersion medium in the gel under the supercritical conditions of the dispersion medium and thereby replacing it with the air while inhibiting drying shrinkage of the gel. However, since enormous costs are needed to realize the supercritical conditions, actual applications of the aerogels obtained by drying under the supercritical conditions are limited to those that are special and worth such high costs. Therefore, a method of drying under atmospheric pressure intended for cost reduction is suggested (Patent Document 4).
There are various applications of the aerogels, including uses as core materials of vacuum thermal insulators or as additives for thermal insulation coatings. In such applications, a shape of an aerogel particle is important. For example, in the application of the aerogel as a core material of vacuum thermal insulators, it is important to reduce contribution of solid conduction (solid heat transfer) in order to further improve the thermal insulation performance of the aerogel, since convection does not contribute to heat transfer. Use of globular particles makes it possible to reduce a contact area (point of contact) between the particles and to therefore decrease heat transfer via contact of the particles. As such, using globular aerogel particles for core materials of vacuum thermal insulators can further improve the thermal insulation performance of the vacuum thermal insulators. In the application of the aerogel as an additive for coating materials as well, making the aerogel particles globular can improve a filling factor of the particles.
There is suggested a method for manufacturing a globular shaped aerogel, including: mixing an acid with an alkali metal silicate by using a mixing nozzle; thereafter spraying a resultant mixture; and making a droplet of the mixture directly into a gel (Patent Document 5).
However, the aerogel having a globular particle shape tends to have a high bulk density due to good fluidity thereof. Since the bulk density and solid heat transfer are generally in a positive correlation with each other, reducing the bulk density is expected to lead to further reduction of solid heat transfer. Therefore, in the application of the aerogel having a globular particle shape as a core material for vacuum thermal insulators or in other applications, reduction of the bulk density is especially desired.